The present invention relates to navigational systems and, more particularly, to a method, apparatus, system, and computer software program product for determining position integrity in a navigational system having a Global Navigation Satellite System (GNSS) component.
Global Navigation Satellite Systems (GNSS) such as, for example, GPS devices, are well known in the art and are commonly used to determine the geodetic latitude and longitude coordinates of mobile vehicles employing such devices. For simplicity, a GPS device will be discussed herein as an example of a GNSS, wherein the terms xe2x80x9cGNSSxe2x80x9d and xe2x80x9cGPSxe2x80x9d may be used interchangeably. However, it will be understood by one skilled in the art that the present invention is not restricted to a GPS device and may be applicable to other GNSS-type devices according to the spirit and scope of the present invention.
With a GPS device, information signals transmitted from a plurality of satellites to a GPS receiver are analyzed using known trilateration techniques in order to determine the geodetic coordinates of the receiver, wherein the geodetic coordinates are typically provided in latitude and longitude. The geodetic coordinates (latitude and longitude), however, may vary in accuracy due to, for example, atmospheric conditions, selective satellite availability, and the relevant positions of the satellites with respect to the line-of sight view of the satellites. Often associated with this variance in GPS accuracy is an integrity determination, which produces a warning if it is determined that the GPS accuracy is insufficient to be relied upon for navigational purposes. Accordingly, where a GPS integrity system is provided, a maximum horizontal position error, otherwise referred to as a xe2x80x9chorizontal protection levelxe2x80x9d (HPL) may be determined and compared to an allowable radial error, otherwise referred to as a xe2x80x9chorizontal alarm limitxe2x80x9d (HAL). If the HPL is found to exceed the HAL, then a warning is issued that the geodetic coordinates should not be relied upon for accuracy.
One method of determining the integrity of a GPS system is the Receiver Autonomous Integrity Monitoring (RAIM) concept which is typically implemented in software in the GPS receiver and which employs an instantaneous self-consistency check during the determination of the geodetic coordinates. In order for RAIM to function as intended, a minimum plurality of satellite signals are required. Where such a minimum plurality of satellite and/or satellite signals are not available, the RAIM internal consistency check may not be available (xe2x80x9cRAIM unavailablexe2x80x9d), where, in turn, no horizontal position integrity information is available. In addition, the RAIM may also generate error values based upon the consistency check, which are then compared to predetermined error limits. Accordingly, should an error value exceed the corresponding error limit, a RAIM alarm may be generated to indicate the failure of the consistency check (xe2x80x9cRAIM alarmxe2x80x9d), where, in other words, horizontal position data may be available, but without integrity. In such instances, where RAIM is not available or a RAIM alarm is generated, the integrity of the geodetic coordinates may be questionable. Thus, there exists a need for a GNSS device capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated.
In some instances, the GPS device may be interfaced with other navigational equipment, wherein the GPS device may also be relied upon to provide location coordinates as well as position integrity information. For example, the GPS device may be interfaced with a Mode S transponder, via a processing unit, with the transponder configured to receive position integrity information from the GPS device as is known in the art. The position integrity information is converted into a corresponding code in a data stream, which is then transmitted by the transponder. The data thus transmitted by the transponder indicates the position of the vehicle carrying the GPS device as well as the level of integrity and/or accuracy of that position information.
An interfaced GPS device may be classified as, for example, a xe2x80x9csole means of navigationxe2x80x9d GPS receiver (xe2x80x9cDO-229A GPS receiverxe2x80x9d) as identified in a document entitled xe2x80x9cMinimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipmentxe2x80x9d, document number RTCA/DO-229A, or a xe2x80x9csupplemental navigationxe2x80x9d device (xe2x80x9cDO-208 GPS receiverxe2x80x9d) as identified in a document entitled xe2x80x9cMinimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS)xe2x80x9d, document number RTCA/DO-208, wherein both documents are incorporated herein by reference in their entirety. Where a xe2x80x9csole means of navigationxe2x80x9d GPS receiver is available to be interfaced with the transponder, a RAIM algorithm in the GPS receiver provides an HPL to the processing unit when RAIM is available and no RAIM alarm is present. However, if RAIM is not available or a RAIM alarm is present, an HPL is not provided to the processing unit and the transponder is thus not able to transmit any position integrity and/or accuracy information. Further, where a xe2x80x9csupplemental navigationxe2x80x9d device is provided, such a device is typically capable of determining geodetic coordinates, but may or may not be configured to execute a RAIM algorithm. Even if a RAIM algorithm is executed by the device, the device is typically not configured to return an HPL to the processing unit since xe2x80x9csupplemental navigationxe2x80x9d devices are not required to be capable of determining an HPL. Thus, there also exists a need for a method of determining position integrity information when a vehicle is equipped with a xe2x80x9csupplemental navigationxe2x80x9d device lacking the capability of determining an HPL. In addition, there exists a further need for a xe2x80x9csole means of navigationxe2x80x9d GPS receiver capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated
Thus, there exists a need for a GNSS device capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated. There also exists a need for a method of determining position integrity information when a vehicle is equipped with a xe2x80x9csupplemental navigationxe2x80x9d device lacking the capability of determining an HPL.
The above and other needs are met by the present invention which, in one embodiment, provides a method for determining position integrity in a system having a Global Navigation Satellite System (GNSS) component. For an alarm limit in a plurality of successive alarm limits, with each alarm limit corresponding to a position integrity level, it is selectively determined whether valid position integrity information is available. At the alarm limit at which valid position integrity information is first available, a corresponding position integrity level is determined. If no valid position integrity information is available for any of the alarm limits, a default position integrity level is then designated.
Another advantageous aspect of the present invention comprises an apparatus for determining position integrity information, and transmitting at least one of position accuracy information and position integrity information, in a system having a GNSS component such as, for example, a Global Positioning System (GPS) device. The apparatus comprises a transponder configured to transmit a type code indicative of position accuracy and/or integrity, a navigational device capable of executing a Receiver Autonomous Integrity Monitoring (RAIM) algorithm to determine whether valid position integrity information is available, and a processing unit in communication with the transponder and the navigational device. The processing unit is configured to selectively provide successive alarm limits to the RAIM algorithm executed by the navigational device, wherein the processing unit is also configured to determine a valid position integrity level corresponding to the alarm limit at which valid position integrity information is first available. The processing unit is further configured to designate a default position integrity level if no valid position integrity information is available for any of the alarm limits. The processing unit is thereafter configured to direct to the transponder at least one of the valid position integrity level, the default position integrity level, and a position accuracy level corresponding to at least one of the valid position integrity level and the default position integrity level, from which the transponder thereafter determines the corresponding type code for transmission.
Still another advantageous aspect of the present invention comprises a system capable of determining position integrity information, and transmitting position accuracy information and/or position integrity information, in an apparatus having a GNSS component such as, for example, a Global Positioning System (GPS) device. The system comprises a computer device having a first processing portion for directing the execution of a RAIM algorithm to determine whether valid position integrity information is available. A second processing portion selectively provides successive alarm limits to the RAIM algorithm so as to determine a valid position integrity level corresponding to the alarm limit at which valid position integrity information is first available. A third processing portion designates a default position integrity level if no valid position integrity information is available for any of the alarm limits. A fourth processing portion then directs to a transponder at least one of the valid position integrity level, the default position integrity level, and a position accuracy level corresponding to at least one of the valid position integrity level and the default position integrity level, from which the transponder thereafter determines the corresponding type code for transmission.
Yet another advantageous aspect of the present invention comprises a computer software program product for determining position integrity information, and transmitting position accuracy information and/or position integrity information, in a system having a GNSS component such as, for example, a Global Positioning System (GPS) device. The computer software program product comprises a first executable portion configured to direct the execution of a RAIM algorithm to determine whether valid position integrity information is available. A second executable portion selectively provides successive alarm limits to the RAIM algorithm so as to determine a valid position integrity level corresponding to the alarm limit at which valid position integrity information is first available. A third executable portion is configured to designate a default position integrity level if no valid position integrity information is available for any of the alarm limits. A fourth executable portion then directs to a transponder at least one of the valid position integrity level, the default position integrity level, and a position accuracy level corresponding to at least one of the valid position integrity level and the default position integrity level, from which the transponder thereafter determines the corresponding type code for transmission.
Thus, embodiments of the present invention provide a method, apparatus, system, and computer software program product for determining position integrity in a system having a GNSS component. Embodiments of the present invention also provide a GNSS device capable of determining the integrity of measured geodetic coordinates in instances where RAIM is not available or a RAIM alarm has been generated. Embodiments of the present invention are further capable of determining position integrity when a vehicle is equipped with a xe2x80x9csupplemental navigationxe2x80x9d device lacking the capability of determining an HPL. Thus, embodiments of the present invention provide distinct advantages over prior art navigational systems having a GNSS component.